This invention relates to masks used in lithography and more particularly to a method of making a mask which can be used for X-ray lithography.
In the manufacture of integrated semiconductor circuits various types of lithography are used in order to expose a photoresist which is deposited on the semiconductor wafer. With requirements for miniaturization, and placing more devices on a given chip, lithography has progressed toward shorter wave lengths. Shorter wave lengths are necessary in order to obtain good resolution and small size. Thus, there has been a progression from optical lithography utilizing the visible spectrum to ultraviolet [UV] lithography and more recently to X-ray lithography, X-rays having a particularly short wave length and permitting particularly fine lines to be defined, such as for example, X-rays in the range of from 4 to 50 angstroms, and more usually in the range of from 4 to 13 angstroms. In such lithography, a mask having the desired pattern thereon is interposed between the source of radiation, e.g. UV or X-ray, and a semiconductor substrate coated with the resist on which the pattern is to be exposed. The mask must result in good definition of the lines being exposed with the masked area opaque to the radiation being used and the mask substrate transparent to that radiation. Because of the small line widths which it is desired to achieve in X-ray lithography, the making of a mask which will provide good resolution becomes a problem.
The typical material used as an absorber on the mask used in X-ray lithography is gold. Although other elements exhibit an increase in soft X-ray absorption, gold is notably easier to work with.
The thickness of the gold absorber is determined by the contrast requirements of the X-ray photoresist. For low sensitivity, high contrast photoresist such as PMMA, 0.2 to 0.3 microns is adequate. However, for the high sensitivity low contrast photoresist which is required for short exposure times, gold thickness in the range of from 0.5 to 0.8 microns is required. Furthermore, if submicrometer features are to be replicated, it is necessary that the gold absorber pattern have a square edge profile. In other words, square edges are essential if small features are packed close together. Since attenuation varies exponentially with thickness, a sloping edge profile will result in loss of edge acuity during X-ray exposure, which can lead to line width control difficulties and photoresist thickness variations where a negative photoresist is used.
The conventionally utilized photolithography and etch techniques use chemical etchings to delineate the gold absorber patterns. These procedures result in undercutting of the photoresist at the pattern edges and sloping edge profiles are generally obtained. Two other known processes, one of wich is a physcial etch technique known as the "lift-off" process and a process known as sputter etching are generally more successful in producing the desired square edge profiles.
In the lift-off technique, a photoresist is deposited on a substrate which is transparent to the X-ray radiation. The photoresist is exposed with the desired pattern and the photoresist developed so as to expose the substrate in the areas where it is desired to deposit gold. This exposure results in an undercutting of the photoresist. Thereafter, a thin adhesion layer of chromium is deposited followed by the deposit of a layer of gold. Success with this technique requires the undercut photoresist profile to prevent a continuous film of gold from covering the steps in the photoresist. Consequently, the gold thickness is limited to about two-thirds of the photoresist thickness and approximately one micron of photoresist is required to define the gold absorber thickness [0.6 microns]. The use of thick photoresist limits the resolution and the minimum line width that can be produced.
The lft-off technique has normally employed photoresist patterns defined by electron beam exposure in order to produce the undercut profile. Although lift-off can be achieved with the square profiles that can be produced by ultraviolet exposure of positive photoresist, there is a tendency to tear the gold at the edges of the feature and hence produce a ragged edge profile. Furthermore, the process cannot be used to define isolated features. An island of photoresist cannot be lifted reliably because there is no access to the photoresist for the solvent.
The other prior art method, i.e., sputter etching, gives a fairly steep angle of about 70.degree. but this is still a limitation. Furthermore, the sputter etching process is much more complex than the lift-off technique.
Additional information concerning the processes under consideration herein and varius prior art proposals may be obtained from the following references: